The present invention relates to acoustic printing, and more particularly to improving the off state of a column switch, in order to control the on/off switching ratio between ejectors of an acoustic printhead.
The fundamentals of acoustically ejecting droplets from an ejector device such as a printhead has been widely described, and the present assignee has obtained patents on numerous concepts related to this subject matter. In acoustic printing, an array of ejectors forming a printhead is covered by a pool of liquid. Each ejector can direct a beam of sound energy against a free surface of the liquid. The impinging acoustic beam exerts radiation pressure against the surface of the liquid. When the radiation pressure is sufficiently high, individual droplets of liquid are ejected from the pool surface to impact upon a medium, such as paper, to complete the printing process. The ejectors may be arranged in a matrix or array of rows and columns, where the rows stretch across the width of the recording medium, and the columns of ejectors are approximately perpendicular.
Ideally, each ejector when activated ejects a droplet identical in size to the droplets of all the other ejectors in the array. Thus, each ejector should operate under identical conditions.
In acoustic printing, the general practice is to address individual ejectors by applying a common RF pulse to a segment of a row, and to control the current flow to each ejector using column switches. In some cases it is desirable to use one column switch for several rows in parallel in order to reduce the number of column driver chips and wire bonds, and hence cost, in the system. Unfortunately, this approach results in parasitic current paths which can cause undesired RF current to flow through ejectors that are not in an ON state.
In existing systems, the switching ratio is limited and will vary with the number of ejectors that are ON in a given row. A switching ratio is defined as the RF power in an OFF ejector, to the RF power in an ON ejector (i.e. POFF/PON).
FIG. 1 illustrates an acoustic switching array with a desired current path for a selected row and selected column for an existing system. Switching matrix 10 is a 4-row 12a, 12b, 12c, 12d by 64 column 14a, 14b, 14zz switching matrix. Rows are connected to the matrix via switching elements 16a, 16b, 16c, 16d, and columns are also connected through switching elements 18a, 18b, 18zz. At the intersection of the columns and rows are transducers 20. Current paths of matrix 10 are terminated at RF ground 21. It is to be appreciated that while the matrix of FIG. 1 is a 4-row by 64-column matrix, the present invention may be used in other matrix designs.
Matrix 10 is supplied by a power source 22 which provides its output to an RF signal matching circuit 24. By proper switch sequencing, a desired current path for a selected row and selected column is obtained. For example, in FIG. 1, by closing switch 16a and switch 18a, a current path is provided from the RF matching network 24 to transducer 20a via row 12a and column 14a. As the remaining rows and columns are unselected, only transducer 20a is intended to be activated to emit a droplet.
Unfortunately, the interconnect paths used to implement a low-cost acoustic printhead include unavailable, undesirable current paths, as shown and discussed for example in connection with FIGS. 2-5. One problem with the proposed printheads is that they used switches which are known as xe2x80x9cleakyxe2x80x9d or xe2x80x9clossyxe2x80x9d switches which add to the existence of undesirable current paths. An example of the foregoing is depicted in FIG. 2. In this figure, switches 16a and 18a are maintained in a closed position while the remaining switches are unselected, and current is provided to transducer 20a. However, undesired current will also flow through transducer 20b, which is in selected row 12a but unselected column 14a. Similarly, FIG. 3 illustrates a situation where undesired current flows through transducer 20c, which is in selected column 18a and unselected row 12c. 
FIGS. 4 and 5 set forth similar simplified depictions of switching matrix 10.
FIG. 4 illustrates a situation where 63 columns 26 and one row 12a are selected, i.e. are ON, and a single column 28 and remaining three rows 12b-12d are unselected, i.e. are OFF. Under this arrangement, the inventors have calculated that there is approximately 514 xcexcA flowing through transducer 30, which represents the transducers in selected row 12a, and 63 ON columns 26 of matrix 10. It was also determined by this analysis that 393 xcexcA of current will flow in transducer 32, located in selected row 12a and the 64th unselected column 28 of transducers. With this information, it is found that the switching ratio between these two currents is equal to:
xe2x80x83393 xcexcA/514 xcexcA=0.765=xe2x88x922.32 dB.
FIG. 5 depicts an alternative arrangement where one column 34, and one row 12a are selected, and remaining 63 columns 36 and 3 rows 12b-12d are unselected. In this situation, the selected current path for transducer 38 has a current of 504 xcexcA, whereas an unwanted current of approximately 368 xcexcA exists through each of the unselected transducers connected to selected column 34 and unselected rows 12b-12d. This results in a switching ratio equal to:
368 xcexcA/504 xcexcA=0.730=xe2x88x922.73 dB.
The cumulative current through switch 18a is approximately 1607 xcexcA (i.e. 504 xcexcA from the transducer in column 34, row 12a, and from the transducers in column 34, rows 12b-12d, at 368 xcexcA each), and the voltage at switches 18b-18zz is 741 mv.
When using aqueous inks for acoustic ink printing, the desired ejection velocity will be approximately 4 m/sec. This can be achieved using approximately 1 dB of power over the ejection threshold. Given that there are power non-uniformities in the aqueous printhead of approximately +/xe2x88x920.5 dB, and the desire to maintain some margin of safety (e.g. xe2x88x920.5 dB) to insure that ejectors which are unselected are truly OFF, an appropriate switching ratio may be found by the restrictions of: switching ratio (SR) greater than (overdrive for 4 m/sec)+(non-uniformity)+(margin to insure appropriate OFF state), which results in:
SRxe2x89xa71+0.5+0.5=xe2x88x922 dB.
Therefore, a switching ratio of xe2x88x922.5 to xe2x88x923.0 dB will be acceptable for printing of aqueous inks, when a xe2x88x920.5 to xe2x88x921,0 dB safety margin is added.
However, and more specifically related to the present invention, phase-change inks require more power over the threshold than aqueous inks. To achieve a necessary 4 m/sec ejection velocity, it has been determined that a xe2x88x924 dB power over the threshold will be required. For phase-change inks, it is intended to use static E-fields to reduce this power requirement, however it is still necessary to eject the droplets at approximately 2 m/sec, i.e. xe2x88x922 dB over threshold. Non-uniformities in the phase-change printhead are similar to those for aqueous ink printheads (i.e. +/xe2x88x920.5 dB), and the margin for turning the switches fully OFF will also be similar (i.e. xe2x88x920.5 dB). Therefore, the switching ratio for phase-change inks will require:
SRxe2x89xa72+0.5+0.5=xe2x88x923 dB.
Then, with a xe2x88x920.5 to xe2x88x921,0 dB safety margin added, a switching ratio of xe2x88x923.5 to xe2x88x924.0 dB is acceptable. Existing switching networks do not insure adequate switching ratios for phase-change printing when the foregoing requirements are taken into consideration.
It has thus been determined desirable to increase the switching ratio, and to control the switching ratio at a desired level, independent of the number of ejectors which are ON. It has also been determined desirable to provide such control in a circuit which is compact, manufacturable, and is functional with the general designs of acoustic printheads.
Two embodiments of column switch compensation circuits are disclosed which act to ensure a necessary level of turnoff for column switches in a transducer matrix. With attention to another aspect of the invention, shown are several integrated semi-conductor architectures for use in a compensation circuit which drives transducers of an acoustic printhead. The architectures disclose switching circuitry which provides for an injection of compensating current in order to improve the turn off an unselected column in a transducer switching array or matrix. The integrated circuits are designed to provide isolation between a column switch, integrated as a high-voltage diode, and a compensation switch, configured as a switching diode or PMOS switch which operates inversely to the column selecting switch. Implementation of the compensation switch ensures a desired turn-off of an unselected column switch associated with an unselected transducer.